The present invention relates to obtaining digital panoramic images and displaying panoramic images on computer screens.
FIG. 1 represents a classical device allowing a digital panoramic image to be produced and presented on a computer screen. The device comprises a digital camera 1 equipped with a panoramic objective lens 2 of the “fish-eye” type, having an angular aperture on the order of 180°. The camera 1 is connected to a computer 5, such as a microcomputer for example, equipped with a screen 6. The connection to the microcomputer 5 may be permanent, when, for example, the camera 1 is a digital video camera, or temporary, when, for example, the camera 1 is a still digital camera equipped with an image memory, the connection then being carried out at the time the image files are to be transferred into the microcomputer.
FIG. 2 schematically represents the appearance of a panoramic image 3 obtained by means of the panoramic objective lens 2. The round appearance of the image is characteristic of the axial symmetry of panoramic objective lenses and the image has dark edges 4 that will subsequently be removed. This digital panoramic image is delivered by the camera 1 in the form of a computer file containing image points coded RGBA arranged in a two-dimensional table, “R” being the red pixel of an image point, “G” the green pixel, “B” the blue pixel, and “A” the Alpha parameter or transparency. The parameters R, G, B, A are generally being coded on 8 bits.
The image file is transferred into the microcomputer 5 which transforms the initial image into a three-dimensional digital image, then presents the user with a sector of the three-dimensional image in a display window 7 occupying all or part of the screen 6.
FIG. 3 schematically shows classical steps of transforming the two-dimensional panoramic image into a panoramic image offering a realistic perspective effect. After removing the black edges of the image, the microcomputer has a set of image points forming an image disk 10 of center O and axes OX and OY. The image points of the image disk are transferred into a three-dimensional space defined by an orthogonal coordinate system of axes O′X′Y′Z, the axis O′Z being perpendicular to the plane of the image disk. The transfer is performed by a mathematical function implemented by an algorithm executed by the microcomputer, and leads to obtaining a set of image points referenced in the coordinate system O′X′Y′Z. These image points are for example coded in spherical coordinates RGBA(φ,θ), φ being the latitude and θ the longitude of an image point. The angles φ and θ are coded in 4 to 8 bytes (IEEE standard). These image points form a hemisphere 11 when the panoramic objective lens used has an aperture of 180°, otherwise a portion of a hemisphere. The microcomputer thus has a virtual image in the shape of a hemisphere one sector 12 of which, corresponding to the display window 7, is presented on the screen (FIG. 1) considering that the observer is on the central point O′ of the system of axes O′X′Y′Z, which defines with the center O″ of the image sector 12, a direction O′O″ called “viewing direction”.
In order to avoid the image sector displayed 12 having geometrical distortions unpleasant for the observer, the classical panoramic objective lenses must have a distribution function of the image points according to the field angle of the object points of a panorama that is as linear as possible. Therefore, if two points A′, B′, situated on the same meridian of the hemisphere 11, and the corresponding points A, B on the image disk 10 are considered, the ratio between the angles (A′O′Z) and (B′O′Z) must be equal to the ratio between the distances OA and OB on the image disk.
Due to this property of linearity of a classical panoramic objective lens, image points corresponding to object points having an identical field angle form concentric circles C10, C20 . . . C90 on the image disk 10, as represented in FIG. 4A. Classically, “field angle of an object point” means the angle of an incident light ray passing through the object point considered and through the center of the panorama photographed, relative to the optical axis of the objective lens. The field angle of an object point can be between 0 and 90° for an objective lens having an aperture of 180°. Therefore, the circle C10 is formed by the image points corresponding to object points having a field angle of 10°, the circle C20 is formed by image points corresponding to object points having a field angle of 20°, etc., the circle C90 being formed by the image points having a field angle of 90°.
FIG. 4B represents the shape of the distribution function Fdc of a classical panoramic objective lens, which determines the relative distance dr of an image point in relation to the center of the image disk according to the field angle ax of the corresponding object point. The relative distance dr is between 0 and 1 and is equal to the distance of the image point in relation to the center of the image divided by the radius of the image disk. The ideal form of the function Fdc is a straight line of gradient K:dr=Fdc(α)=Kαin which the constant K is equal to 0.111 degree−1 (1/90°).
This technique of displaying a digital panoramic image sector on a computer screen has various advantages, particularly the possibility of “exploring” the panoramic image by sliding the image sector presented on the screen to the left, the right, upwards or downwards, until the limits of the panoramic image are reached. This technique also allows complete rotations of the image to be carried out when two complementary digital images have been taken and supplied to the microcomputer, the latter thus reconstituting a complete panoramic sphere by assembling two hemispheres. Another advantage provided by presenting a panoramic image on screen is to enable the observer to make enlargements or zooms on parts of the image. The zooms are performed digitally, by shrinking the image sector displayed and expanding the distribution of the image points on the pixels of the screen.
Various examples of interactive panoramic images can be found on the Web. Reference could be made in particular to the central site “http://www.panoguide.com” (“The Guide to Panoramas and Panoramic Photography”) which gives a full overview of all the products available to the public to produce these images. Software programs allowing digital panoramic photographs to be transformed into interactive panoramic images are offered to the public in the form of downloadable programs or CD-ROMs available in stores.
Despite the various advantages that this technique for displaying digital images offers, the digital enlargements have the disadvantage of being limited by the resolution of the image sensor used when taking the initial image and the resolution of an image sensor is generally much lower than that of a classical photograph. Therefore, when the enlargement increases, the granulosity of the image appears as the limits of the resolution of the image sensor are being reached.
To overcome this disadvantage, it is well known to proceed with pixel interpolations so as to delay the apparition of the blocks of color which betray the limits of the resolution of the sensor. However, this method only improves the appearance of the enlarged image sector and does not in any way increase the definition. Another obvious solution is to provide an image sensor with a high resolution, higher than the resolution required to present an image sector without enlargement, so that there is a remaining margin of definition for zooms. However, this solution is expensive as the cost price of an image sensor rapidly rises with the number of pixels per unit of area.
Some attempts have been made to improve the quality of the enlargements, by changing the optical properties of the panoramic objective lenses themselves. Thus, U.S. Pat. No. 5,710,661 teaches capturing a panoramic image with two overlocking objective lenses using a set of mirrors. A first set of mirrors provides an overall view, and a mobile central mirror provides a detailed view on a determined zone of the panorama. However, this solution does not offer the same flexibility as digital zooms, particularly when the image is not displayed in real time, as the observer no longer has the possibility of choosing the image portion that he wants to enlarge once the photograph has been taken.